1. Field of the Invention
The present invention relates to a transformer and an inductor which are used in a switching regulator, and more particularly to those using a split-type bobbin.
2. Discussion of the Background
It is conventionally known that, in various transformers and inductors for switching power supplies, leakage flux to the outside occurs during the operation of the transformer and the inductor. To shield this leakage flux, a method is generally taken in which a copper plate shield ring is provided on side surfaces of the transformer via outer peripheral portions of a core, as shown in FIG. 6. In addition, as shown in Japanese Utility Model Registration No. 2518250 and Japanese Utility Model Registration No. 2518241, methods are disclosed in which a shielding effect similar to that of the aforementioned copper plate shield ring is obtained by forming a shield coil making use of a wire without using the copper plate shield ring.
With the above-described conventional transformer and inductor, the following problems were encountered.
Although the manufacture of the transformers and inductors is generally automated, in the manufacturing method for providing the copper plate shield ring on the side surfaces of the transformer and the inductor as shown in FIG. 6, the fitting and soldering in a manual operation are still the mainstream, and the number of manufacturing steps is large, and the simplification of the process and automation are hampered.
Although a method has been proposed in which, in FIG. 7, instead of the copper plate shield ring a chamber is provided around a bobbin upper collar portion 70, and a shield coil 74 is formed by a wire. In this structure, a winding start and a winding end of the shield coil are connected to a terminal 71 embedded in the bobbin upper collar portion 70 so as to form a short-circuited ring. Accordingly, soldering processing of the terminal 71 is required, and in order to complete the transformer, two times of soldering processing including that for opposite-side bobbin terminals 79 is performed. Accordingly, the number of manufacturing steps increased, which is not desirable.
In FIG. 8, on the other hand, a shield coil 84 in the form of a short-circuited ring is formed by a wire, and after the core and the bobbin are combined, the shield coil 84 is fitted in two shield-coil receiving portions 81 formed in a bobbin upper collar portion 80. Hence, there is a problem in the stability of the shield coil 84, and the step of fabricating the shield coil is separately required, which is not desirable.
In the above-described structure of the shield coil using the wire, the direction of leakage flux which can be shielded is only the vertical direction of the transformer and the inductor, and an effect is not obtained with respect to the leakage flux on the overall side surfaces of the core outer legs.